Lithium ion batteries or battery systems have established themselves in practically every field of mobile energy storage thanks to their high energy and power density, whether it be pedelecs, power tools, hybrid drives, electric vehicles, or even railway applications. At the same time, lithium ion battery systems are becoming increasingly important for stationary energy storage systems. Many lithium ion cells or their cell chemistry are characterized by a flat curve of their state of charge versus voltage. In the marginal regions below around 10% and above 90% state of charge (SoC), however, the voltage level of the battery cells changes very quickly. Over time, the battery cells might drift apart in their voltage level. This may occur due to various effects, such as different rates of self-discharge, inhomogeneous temperatures in operation or also premature aging of certain cells in a battery grouping, or also due to different lots (delivery from different manufacturing lots). A battery system made from such a cell grouping should only be discharged or charged until a battery cell has reached an upper or lower threshold value of the voltage. Battery management systems (BMS) therefore also have the task of counteracting this drifting apart of the cell voltages in a battery—in this context, one speaks of a “balancing”. This is employed in order to preserve the complete usable capacitance of a battery or a battery system.
Without “cell balancing”, the “weakest” battery cell in a multi-cell battery system determines the capacitance of the overall system. Furthermore, the “weakest” battery cell determines how much energy can be taken up or delivered. This is especially relevant for high-voltage batteries in which a plurality of battery cells are hooked up in series in order to achieve a corresponding overall voltage. By a high-voltage battery in a vehicle is usually meant a battery with a voltage higher than 60 volts, and depending on the purpose of use the chosen voltage may amount to several hundred volts. At present, battery management systems for batteries are usually designed so that passive battery cell balancing ensures a balanced state in regard to the battery cell voltages or battery capacitances. Due to the fact that usually very many battery cells are connected in series in a battery, it is of course very important for all battery cells to be equally loaded—if possible—in the case of a bidirectional current loading of all battery cells (series circuit).
One possibility is to provide a resistive bypass across each individual battery cell, which can be controlled so that any given portion of the charge current will bypass the battery cell. The drawback is that in this case a disproportionately large portion of charge energy is transformed into heat, until such time as all battery cells are fully charged, and that the “balancing” in theory only works during charging mode and does not work during the discharging. Also a balancing occurs during standby of a battery, which may lead to a gradual discharging of the battery. Among stationary applications, active charge balancing devices are known which also provide a balancing among individual battery cells during discharge mode. These DC/DC converters, realized on the basis of transformers, on the one hand require a complex and costly circuitry and lead to both a larger space requirement and a greater weight, which is a particularly negative consideration for mobile applications.